FIG. 10 is a perspective view of a detection element of an angular velocity sensor as one type of a conventional piezoelectric device disclosed in Patent Document 1, and FIG. 11 is a cross-sectional view taken along line 11-11 of FIG. 10.
The conventional angular velocity sensor includes tuning fork type detection element 1 shown in FIG. 10 and a signal processing circuit (not shown) for processing a signal output from detection element 1 and calculating an angular velocity. As shown in FIG. 10, detection element 1 is configured by a tuning fork type in which a pair of facing arms 2 is connected by connection portion 3, and connection portion 3 is mounted in a mounting substrate and is used. Driving portions 4 for driving arms 2, detecting portion 5 for outputting an angular velocity signal generated due to the angular velocity applied to detection element 1, and monitor portion 6 for monitoring the driving state of detection element 1 are arranged in each of the pair of arms 2. In the arrangement, two driving portions 4 are arranged with one detecting portion 5 interposed therebetween and monitor portion 6 is arranged in the vicinity of the boundary between each of arms 2 and connection portion 3, in the opposite directions of arms 2.
As shown in FIG. 11, each of the pair of arms 2 has silicon substrate 9 having two layers including silicon layer 7 and silicon oxide layer 8 obtained by oxidizing the surface thereof, and driving portions 4 and detecting portion 5 are formed on silicon substrate 9 with intermediate layer 10 interposed therebetween. Each of driving portions 4 and detecting portion 5 includes lower electrode layer 12 and upper electrode layer 13 with piezoelectricity layer 11 interposed therebetween. Adhesion layer 14 is formed between piezoelectricity layer 11 and upper electrode layer 13.
Intermediate layer 10 is made of titanium (Ti), and lower electrode layer 12 is made of Pt—Ti mainly made of platinum (Pt) containing Ti. Piezoelectricity layer 11 includes two layers including alignment control layer 15 mainly made of lead titanate and PZT layer 16 made of lead zirconate titanate and laminated on alignment control layer 15. Adhesion layer 14 is made of Ti, and upper electrode layer 13 is made of Au.
Recently, the angular velocity sensor has been used in extremely high-temperature environments such as the inside of an engine room as well as a cabin of a vehicle which a person gets in and out of. Accordingly, the reliability of the angular velocity sensor needs to be further improved at a high temperature.
If conventional detection element 1 having the above configuration is used at a high temperature as in the vicinity of an engine, interaction occurs between Ti constituting adhesion layer 14 formed between piezoelectricity layer 11 and upper electrode layer 13 of detection element 1 and PZT layer 16 constituting piezoelectricity layer 11. Accordingly, the dielectric constant, the specific resistance or the piezoelectric constant of the interface of piezoelectricity layer 11 is changed with time and detection sensitivity is changed.
FIG. 12 is a characteristic diagram showing a change in the basic point voltage fluctuation of the same angular velocity sensor at a high temperature with the elapse of time. In the angular velocity sensor using detection sensor 1, as shown in FIG. 12, as an operation time at a high temperature (an operation of 5 V at 125 degrees Celsius) is increased, a basic point voltage which is a voltage generated in the angular velocity sensor when an angular velocity is not applied is significantly changed.
Accordingly, in the conventional piezoelectric device, as can be seen from the characteristic diagram showing an output voltage change against an angular velocity change of the angular velocity sensor of FIG. 13, a voltage when an angular velocity is not generated is changed from the basic point voltage (point V0) to, for example, point V1 of a minus side, and an error of A° C./sec occurs.
However, in the piezoelectric vibrator field, a vibrator is composed of a piezoelectric substance, electrodes are respectively formed on two opposite surfaces of the piezoelectric substance, and tungsten is used as the underlying layers of the electrodes. In this piezoelectric vibrator, in order to prevent the piezoelectric substance as well as the electrodes from being trimmed when the electrodes are laser-trimmed in order to adjust a resonance frequency, tungsten, a metal with a high melting point, is used as the underlying layers of the electrodes (see Patent Document 2).
Although it relates to the semiconductor field, there is a technology that, in order to form a T-shaped electrode on a semiconductor substrate, tungsten in which an α phase and a β phase coexist and α-phase tungsten, and thereon tungsten having the α phase are formed on the semiconductor substrate and processed by etching. In this technology, the T-shaped electrode is obtained by focusing on a difference in the etching rates of the α-phase tungsten and the β-phase tungsten, forming tungsten in which an α phase and a β phase coexist with a higher etching rate on a lower side and forming the α-phase tungsten with a lower etching rate on an upper side (see Patent Document 3).    [Patent Document 1] Japanese Patent Unexamined Publication No. 2005-249645    [Patent Document 2] Japanese Patent Unexamined Publication No. 58-188916    [Patent Document 3] Japanese Patent Unexamined Publication No. 64-30273